The present invention relates generally to crystal oscillators, and more specifically to low-noise, high-stability crystal oscillators.
Crystal oscillators are extremely useful circuits. They provide clocks and periodic signal sources for telecommunications, wired and wireless networks, and myriad other electronic applications. For example, crystal oscillators are commonly used to time data transfers between integrated circuits. In these applications, crystal oscillator phase noise and jitter degrades performance, causes data transmission errors, and limits data throughput. Thus, it is desirable to provide crystal oscillators having low-noise and high-stability.
The signal-to-noise ratio for a crystal oscillator can be improved by increasing its signal strength. One way to increase signal strength or amplitude is to generate a differential signal, as opposed to a single-ended signal. A differential signal not only provides a signal that is nominally twice the amplitude of a single-ended signal, but provides a level of common-mode rejection as well, which further reduces noise. Also, a buffer receiving these larger oscillator signals can operate at a lower gain resulting in less noise.
Unfortunately, excessively large crystal oscillator signals can cause jitter or instability in the oscillator circuit. As these signals become excessive, they may become limited by one or both of a pair of supply voltages for the crystal oscillator. Specifically, electrostatic discharge (ESD) diodes to these supplies can begin to conduct current. This clips the oscillator signals, which adds harmonics and spurious frequency components to the otherwise single-tone signal. These harmonics pull or shift the oscillator operating frequency, resulting in center frequency inaccuracies.
Also, signals from crystal oscillators typically need to be AC coupled to an integrated circuit that is using the oscillator. If the DC level of the crystal oscillator signals could be well controlled, it would be possible to design an input buffer that could directly connect to the crystal without using the AC coupling capacitors. This would reduce component count, save board space, and reduce costs. This would also help prevent the oscillator signals from being clipped by the ESD diodes.
Thus, what is needed are circuits, methods, and apparatus that provide crystal oscillators having large, amplitude-controlled differential signal outputs and mechanisms for controlling their DC levels.